Various minimally invasive and noninvasive medical procedures use targeted applications of energy to increase the temperature of tissue and form heat-induced lesions at a treatment site in the tissue. High-intensity focused ultrasound (“HIFU”) waves, for example, can be propagated into tissue toward a discrete focal region, and the accumulation of the resultant harmonic frequencies can induce rapid heating at the focal region that ablates, necrotizes, and/or otherwise damages the tissue. In a clinical setting, HIFU-induced heating can be used to treat benign and malignant tumors (e.g., in the brain, uterus, prostate, liver, etc.) and/or occlude blood vessels (e.g., to induce hemostasis of internal bleeds, intervene in fetal blood sharing anomalies, and confine tumor blood supply). During HIFU therapy and/or other treatments that form heat-induced lesions, image guidance and treatment monitoring (e.g., temperature monitoring) can be used for controlling and optimizing the parameters of the treatment and assessing its efficacy.
HIFU therapies are typically used with magnetic resonance imaging (“MRI”) to monitor tissue temperature and/or provide image guidance. For example, MRI can be used to map a temperature profile of a treatment site with a relatively high spatial resolution (e.g., approximately to 2 mm2), determine treatment volumes (e.g., magnetic resonance images indicate coagulated tissue size and location above a threshold thermal dose), provide post-treatment verification, and follow post-treatment tissue repair. Although MRI has relatively good spatial and temperature resolutions, MRI has a limited time resolution that is inadequate for motion compensation during therapies and causes misregistration. In addition, MRI cannot estimate temperatures above approximately 65° C. (when tissues begin to denature and proton relaxation becomes dominant), therefore making it unsuitable for HIFU therapies that induce focal temperatures ranging from approximately 60-90° C. Moreover, the high costs associated with MRI limit its availability to patients and physicians.
Ultrasound-based monitoring methods can also be used for treatment monitoring during HIFU therapies, and can be significantly less expensive than MRI-based methods. Unlike MRI, ultrasound-based monitoring has a relatively high temporal resolution (e.g., tens of Hertz). Some HIFU systems use B-mode ultrasound for treatment monitoring, such as HIFU devices made by Chongqing Haifu Technology, Co. of Barcelona, Spain and the Sonoblate® 500 made by Focus Surgery, Inc. of Indianapolis, Ind. During a HIFU therapy, HIFU-induced bubble formations increase backscatter and produce a hyperechoic region that correlates to the HIFU focal region. B-mode ultrasound can use this hyperechogenicity to help direct therapy and measure tissue coagulation. However, B-mode ultrasound systems cannot use the induced hyperechogenicity to provide a direct indication of tissue temperature or damage. In other types of ultrasound-based monitoring, radiofrequency (RF) signals from diagnostic ultrasound scanners are used to form maps based on temperature-induced changes in tissue resonances. However, RF-based maps are currently limited to temperatures well below the coagulation threshold, and therefore cannot be used during HIFU or other high-temperature therapies. Various other ultrasound-based temperature monitoring techniques have also been explored. For example, elastography-based temperature estimation methods can use raw ultrasound RF data from images obtained before and after external tissue compression to image HIFU lesions based on changes in tissue stiffness. Other elastography-based monitoring systems, such as ultrasound-stimulated acoustic emission methods, map tissue temperature by generating low frequency radiation forces. The radiation forces have amplitudes that depend on tissue stiffness and absorption (i.e., acoustic emissions vary linearly with temperature), but only for temperatures below the coagulation threshold (when then the strain-temperature relationship is abolished). “Before and after” ultrasound RF data has been used to form images based on HIFU-induced changes, and is therefore insensitive to tissue inhomogeneities. These and other ultrasound-based temperature estimation methods (e.g., acoustic radiation force imaging) fail to provide real-time temperature mapping for the temperatures reached during HIFU therapies and other therapies that form heat-induced lesions.